Chapter IV.--To Prevent Ambiguity, We Must Begin with Clear Definition.
For every question is solved from pre-existing knowledge. And the knowledge pre-existing of each object of investigation is sometimes merely of the essence, while its functions are unknown (as of stones, and plants, and animals, of whose operations we are ignorant), or [the knowledge] of the properties, or powers, or (so to speak) of the qualities inherent in the objects. And sometimes we may know some one or more of those powers or properties,--as, for example, the desires and affections of the soul,--and be ignorant of the essence, and make it the object of investigation. But in many instances, our understanding having assumed all these, the question is, in which of the essences do they thus inhere; for it is after forming conceptions of both--that is, both of essence and operation--in our mind, that we proceed to the question. And there are also some objects, whose operations, along with their essences, we know, but are ignorant of their modifications.
Such, then, is the method of the discovery [of truth]. For we must begin with the knowledge of the questions to be discussed. For often the form of the expression deceives and confuses and disturbs the mind, so that it is not easy to discover to what class the thing is to be referred; as, for example, whether the foetus be an animal. For, having a conception of an animal and a foetus, we inquire if it be the case that the foetus is an animal; that is, if the substance which is in the foetal state possesses the power of motion, and of sensation besides. So that the inquiry is regarding functions and sensations in a substance previously known. Consequently the man who proposes the question is to be first asked, what he calls an animal. Especially is this to be done whenever we find the same term applied to various purposes; and we must examine whether what is signified by the term is disputed, or admitted by all. For were one to say that he calls whatever grows and is fed an animal, we shall have again to ask further, whether he considered plants to be animals; and then, after declaring himself to this effect, he must show what it is which is in the foetal state, and is nourished.
For Plato calls plants animals, as partaking of the third species of life alone, that of appetency.  But Aristotle, while he thinks that plants are possessed of a life of vegetation and nutrition, does not consider it proper to call them animals; for that alone, which possesses the other life--that of sensation--he considers warrantable to be called an animal. The Stoics do not call the power of vegetation, life.
Now, on the man who proposes the question denying that plants are animals, we shall show that he affirms what contradicts himself. For, having defined the animal by the fact of its nourishment and growth, but having asserted that a plant is not an animal, it appears that he says nothing else than that what is nourished and grows is both an animal and not an animal.
Let him, then, say what he wants to learn. Is it whether what is in the womb grows and is nourished, or is it whether it possesses any sensation or movement by impulse? For, according to Plato, the plant is animate, and an animal; but, according to Aristotle, not an animal, for it wants sensation, but is animate. Therefore, according to him, an animal is an animate sentient being. But according to the Stoics, a plant is neither animate nor an animal; for an animal is an animate being. If, then, an animal is animate, and life is sentient nature, it is plain that what is animate is sentient. If, then, he who has put the question, being again interrogated if he still calls the animal in the foetal state an animal on account of its being nourished and growing, he has got his answer.
But were he to say that the question he asks is, whether the foetus is already sentient, or capable of moving itself in consequence of any impulse, the investigation of the matter becomes clear, the fallacy in the name no longer remaining. But if he do not reply to the interrogation, and will not say what he means, or in respect of what consideration it is that he applies the term "animal" in propounding the question, but bids us define it ourselves, let him be noted as disputatious.
But as there are two methods, one by question and answer, and the other the method of exposition, if he decline the former, let him listen to us, while we expound all that bears on the problem. Then when we have done, he may treat of each point in turn. But if he attempt to interrupt the investigation by putting questions, he plainly does not want to hear.
But if he choose to reply, let him first be asked, To what thing he applies the name, animal. And when he has answered this, let him be again asked, what, in his view, the foetus means, whether that which is in the womb, or things already formed and living; and again, if the foetus means the seed deposited, or if it is only when members and a shape are formed that the name of embryos is to be applied. And on his replying to this, it is proper that the point in hand be reasoned out to a conclusion, in due order, and taught.
But if he wishes us to speak without him answering, let him hear. Since you will not say in what sense you allege what you have propounded (for I would not have thus engaged in a discussion about meanings, but I would now have looked at the things themselves), know that you have done just as if you had propounded the question, Whether a dog were an animal? For I might have rightly said, Of what dog do you speak? For I shall speak of the land dog and the sea dog, and the constellation in heaven, and of Diogenes too, and all the other dogs in order. For I could not divine whether you inquire about all or about some one. What you shall do subsequently is to learn now, and say distinctly what it is that your question is about. Now if you are shuffling about names, it is plain to everybody that the name foetus is neither an animal nor a plant, but a name, and a sound, and a body, and a being, and anything and everything rather than an animal. And if it is this that you have propounded, you are answered.
But neither is that which is denoted by the name foetus an animal. But that is incorporeal, and may be called a thing and a notion, and everything rather than an animal. The nature of an animal is different. For it was clearly shown respecting the very point in question, I mean the nature of the embryo, of what sort it is. The question respecting the meanings expressed by the name animal is different.
I say, then, if you affirm that an animal is what has the power of sensation and of moving itself from appetency, that an animal is not simply what moves through appetency and is possessed of sensation. For it is also capable of sleeping, or, when the objects of sensation are not present, of not exercising the power of sensation. But the natural power of appetency or of sensation is the mark of an animal. For something of this nature is indicated by these things. First, if the foetus is not capable of sensation or motion from appetency; which is the point proposed for consideration. Another point is; if the foetus is capable of ever exercising the power of sensation or moving through appetency. In which sense no one makes it a question, since it is evident.
But the question was, whether the embryo is already an animal, or still a plant. And then the name animal was reduced to definition, for the sake of perspicuity. But having discovered that it is distinguished from what is not an animal by sensation and motion from appetency; we again separated this from its adjuncts; asserting that it was one thing for that to be such potentially, which is not yet possessed of the power of sensation and motion, but will some time be so, and another thing to be already so actually; and in the case of such, it is one thing to exert its powers, another to be able to exert them, but to be at rest or asleep. And this is the question.
For the embryo is not to be called an animal from the fact that it is nourished; which is the allegation of those who turn aside from the essence of the question, and apply their minds to what happens otherwise. But in the case of all conclusions alleged to be found out, demonstration is applied in common, which is discourse (logos), establishing one thing from others. But the grounds from which the point in question is to be established, must be admitted and known by the learner. And the foundation of all these is what is evident to sense and to intellect.
Accordingly the primary demonstration is composed of all these. But the demonstration which, from points already demonstrated thereby, concludes some other point, is no less reliable than the former. It cannot be termed primary, because the conclusion is not drawn from primary principles as premisses.
The first species, then, of the different kinds of questions, which are three, has been exhibited--I mean that, in which the essence being known, some one of its powers or properties is unknown. The second variety of propositions was that in which we all know the powers and properties, but do not know the essence; as, for example, in what part of the body is the principal faculty of the soul.